High-speed digital signal processing has recently been suggested for use in conjunction with coherent detection to allow demodulation of various modulation formats. One major advantage of using digital signal processing after sampling of the outputs from a phase-diversity receiver is that hardware optical phase locking can be avoided and only digital phase-tracking is needed as described in M. G. Taylor, “Coherent detection method using DSP for demodulation of signal and subsequent equalization of propagation impairments,” IEEE Photon. Technol. Lett., vol. 16, pp. 674-676, (February 2004) and R. Noe, “PLL-free synchronous QPSK polarization multiplex/diversity receiver concept with digital I&Q baseband processing,” IEEE Photon. Technol. Lett. vol. 17, 887-889, (April 2005).
Digital signal processing algorithms can also be used to mitigate degrading effects of optical fiber such as chromatic dispersion and polarization-mode dispersion. As suggested in J. H. Winters, “Equalization in coherent lightwave systems using a fractionally spaced equalizer,” J. Lightwave Technol., vol. 8, pp. 1487-1491, (October 1990), for a symbol rate of T, a T/2 tap delay finite impulse response filter may be used to reverse the effect of first order fiber chromatic dispersion. The number of finite impulse response taps required grows linearly with increasing dispersion. As reported in S. J. Savory, “Digital filters for coherent optical receivers”, Optics Express, Vol. 16, Issue 2, pp. 804-817 (January 2008), the number of taps required to compensate for 1000 ps/nm of dispersion (assuming a signal bandwidth of B, and 2 samples per symbol), is N=0.032B2. At long propagation distances the added power consumption required for this task becomes significant. Moreover, a longer finite impulse response filter introduces a longer delay and requires more area on a digital signal processing chip.
Alternatively, an infinite impulse response filter may achieve similar performance with substantially reduced number of operations. This leads to lower power consumption and a smaller device footprint and the use of infinite impulse response filtering for dispersion compensation (DC). In contrast to optical DC which processes bandpass signals, digital finite impulse response or infinite impulse response DC processes baseband signals which is described in C. K. Madsen and J. H. Zhao, Optical Filter Design and Analysis: A Signal Processing Approach, New York: John Wiley & Sons, (1999).
Compared to finite impulse response filtering, infinite impulse response filtering achieves dispersion compensation using a significantly smaller number of taps. Infinite impulse response filtering achieves performance similar to the finite impulse response filtering.